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Middle Aperture

Small Aperture - lots of room for error

Large Aperture - no room for error

INTRODUCTION

Focus errors
limit resolution at
larger apertures more than the differences between lenses.

In real photography one never gets the full resolution
of which large aperture lenses are capable in lab tests.

At large apertures
like f/1.4 and f/2.0 you'll miss most of the performance of which your
lens is capable if your cameras and eyeballs aren't well calibrated,
even for flat, fixed subjects.

SPECIFICS

The graph shows
the maximum possible resolution based on the accuracy of your focus.
It's exactly like depth-of-field. If I wasn't so lazy I would
have drawn a few different graphs, instead I stretched and squeezed
my hand-drawn graph in Photoshop, so please excuse the fat wiggly
lines in the one graph.

Perfect resolution
only happens with perfect focus. No matter how good your lens, if
your focus is just a little bit off your resolution plummets.

Just like depth
of field, this is much pickier at larger apertures.

At larger apertures,
even small amounts of defocus make all lenses have the same performance.
This is why Zeiss and Contax and the Germans do funny things like
design cameras with vacuum film backs to hold film flat.

Most subjects
aren't flat so you can't get perfect focus on your entire subject.
This only matters for testing lenses and for spy satellites.

PRACTICAL
APPLICATIONS

Auto and manual
focusing is only so accurate. It's fine for small apertures like f/5.6,
OK for f/2.8, and usually at it's limits at f/2.0.

When I run tests
I make many shots at slightly different focuses to ensure I catch the
perfect focus. This isn't important
with f/5.6 zooms, but critical with f/1.4 primes.

Back
in film days I had to custom calibrate each of my Nikons so the image
on film matched what I saw through the finder at f/1.4. It made no
difference at f/2.8, but made a world of difference at f/1.4. At f/1.4
the depth of field is so paper-thin that the region of sharpness rarely
was where I wanted it.

Manual
focus is even sloppier on digital SLRs. Today Nikon only has a single
dot that lights up when you're sort of in focus. In the older days
Nikon had two or three lights with arrows to tell you when you were
as exact as it could measure. Today the green dot is only "close
enough," not
perfect. Think about it: if the dot only lit when you were perfect,
it would almost never light and you'd focus right past it! The dot
is OK for slow f/5.6 zooms, but too sloppy for f/1.4 primes.

In my 50mm
lens comparison it became painfully obvious that the range
of focus over which the
"in focus" dot lit on my Nikon
D200 was too broad for f/1.4 lenses. It also wasn't perfectly calibrated.
I discovered that I got best focus when I turned the focus ring slowly
towards closer distances, and stopped right as the dot was on the hairy
edge between off and on on the closer side.

RECOMMENDATIONS

Don't worry about
your lenses' sharpness. The lens designers already have. The lenses
you already have are all you need.